The present invention relates to the field of thermal recording or printing and, more specifically, to a thermally sensitive recording system and method for recording a grey scale or tonal image on a thermally sensitive recording medium of the transparency type.
Commonly assigned, copending application Ser. No. 676,502 filed on Nov. 29, 1984 by Irving Erlichman and entitled "Thermal Recording System and Method", discloses and claims a system and method for recording an image defined by electronic image signals on a thermally sensitive recording medium or paper comprising an opaque base or support layer and a thermally sensitive recording layer carried on one side of the base layer.
The image to be recorded is defined by a matrix array of minute pixel areas, each of which has a desired or target density specified by the electronic image signals. Variations in recorded pixel density is achieved by varying the size of a dot that is recorded in each of a plurality of selected pixel areas on the medium to provide a grey scale image in a manner that is analogous to half-tone lithographic printing.
Image quality, therefore, depends on precisely controlling the size of the recorded dots. To achieve precise control, the recording system is configured for closed loop operation wherein dot size or pixel density is monitored during recording with an electro-optical device such as a photodetector.
A dot is recorded by applying thermal energy to the recording medium which causes an invisible dye composition in the recording layer to turn dark or visible when the applied heat exceeds a threshold dye reaction temperature. Dot size increases with increased amounts of thermal energy applied to form a dot.
The opaque base layer of the medium serves as a contrasting background against which the recorded dots may be viewed by reflected light. In one embodiment of the recording system, a multi-element thermal print head is used to apply thermal energy to the back side of the paper for transmission through the base layer to the recording layer. This allows dot formation to be monitored with a photodetector array facing the recording layer on the front side of the paper where its view is not obstructed by the print head.
In accordance with the electronic image signals, an initial pulse of thermal energy is applied to selected pixel areas to form in each a dot having an initial size which is smaller than needed to achieve target or desired density. The photodetector array measures the density of the pixel areas having initial dots therein and feeds this information back to a control system which compares monitored density to desired density and provides comparison value signals. These comparison signals are used to trigger an additional application of thermal energy to further increase dot size. Again, pixel density is monitored and compared to desired density. The heating and monitoring cycle continues to progressively increase dot size until a predetermined density comparison value is achieved whereupon further application of thermal energy is terminated.
The key to controlling pixel density resides in the ability to accurately monitor the recorded dots with the photodetector array. By applying heat to the back side of the paper, the recorded information is not covered by the print head which facilitates monitoring. Also, the opaque base of the thermal paper provides a contrasting light reflective background which also facilitates obtaining accurate pixel density measurements with the photodetector array.
Recording an image on an opaque base medium provides a "hard copy" or print that is viewed by reflected light. However, there are applications in which it is highly desired to record an image on a transparency type thermally sensitive recording medium. For example, making a "hard copy" of a medical X-ray from electronically recorded image signals, or making overhead projection slides depicting graphic and/or text information for presentation at business meetings.
Transparency type thermally sensitive recording media are commercially available and generally comprise a transparent film or base layer having a transparent thermally sensitive recording layer coated on one side thereof.
Attempts have been made to record images on such a transparency type of medium utilizing the closed loop thermal recording system described above, but the results generally were inferior to those obtained with an opaque base paper.
The reason for this is attributable to erroneous pixel density readings from the photodetector. When the photodetector "looks at" a pixel area to monitor dot formation, it not only "sees" the recorded dot, but looking through the transparent area around the dot, it also sees whatever happens to be in the background of the opposite side of the medium. Unlike the opaque base paper which provides a uniform contrasting background against which the dots are viewed to measure pixel density, the transparent nature of this medium makes it very difficult to obtain consistent and reliable light level readings.
For example, the print head may consist of a linear array of individually addressable resistive heating elements, each having a size that is about the same as a corresponding pixel area on the medium. Suppose this head is pressed against the recording layer side of the transparent medium and a photodetector array is located on the opposite side in alignment with the head. The photodetector looks through the transparent base and recording layers and initially sees a corresponding ones of the heating elements which tend to be rather dark in tone. When heat is applied and a dark dot is formed, the photodetector views it against the dark heating element background which makes it very difficult, if not impossible, to obtain an accurate indication of dot size which in turn determines pixel density.
U.S. Pat. Nos. 3,577,137; 4,355,318; 4,407,003; 4,412,229; 4,442,342; and 4,064,205 are representative examples of prior art thermal printers which sense or calculate various printing parameters and use this information to improve control over the thermal recording process. However, none of these disclosures address the problem of how to avoid erroneous density readings when recording on a transparency type of recording medium.
Therefore it is an object of the present invention to provide a thermally sensitive recording system and method which is especially well suited for recording a grey scale image on a transparency type thermally sensitive recording medium.
It is yet another object to provide such a system and method which enhances the accuracy of monitoring pixel density while image recording is in progress.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.